Conventionally an electronic shutter control method referred to as a so-called “rolling shutter” is used, for example, as an image sensing control method of a CMOS-type image sensing element. In an image sensing operation using the rolling shutter, first, a plurality of pixels that were arranged two-dimensionally are scanned sequentially in sectional units such as lines, to read an electric charge from each pixel and reset each pixel. After the lapse of a predetermined time (charge accumulation period or exposure time) that starts immediately after the reset, the plurality of pixels are rescanned in a similar manner to when they were reset, thereby reading a charge from each pixel, and the read charge (image signal) is output. When sensing a moving image, the above described operation is performed in one-frame periods.
FIG. 7 is an explanatory diagram that illustrates the concept of this conventional image sensing operation, which shows a case in which scanning was performed by line. The slanting broken line shows the scanning position of the reset line, and the slanting solid line shows the scanning position of the read line of the image signal. Accordingly, the interval between the reset line and the read line is the charge accumulation period.
As will be understood from FIG. 7, when sensing an image using the rolling shutter as described above, there is a time difference of one frame between the timing for scanning the first pixel line and the timing for scanning the last pixel line. More specifically, in an image of one frame, scenes with a time difference between the top and bottom of the frame coexist. Therefore, when controlling an image sensing element using the rolling shutter system to sense a moving object, the image is distorted between the top and bottom of the image.
In order to solve this problem, for example, the method disclosed in Japanese Patent Laid-Open No. 2004-140479 has been proposed. According to this method, first, a reset operation and reading operation are performed at a faster rate than the frame rate, and the image data that was read at a high speed is temporarily stored in a storage unit. Thereafter, the stored data is read out in accordance with the frame rate to thereby lessen image distortion.
Meanwhile, as an autofocusing operation, methods are known that moves a focusing lens to a plurality of positions to perform image sensing in each position, and then determine an in-focus position based on a series of AF evaluation values that are obtained by calculation based on the sensed signals. This type of methods includes a method that performs image sensing after stopping the lens after the focusing lens has been moved, and a method that performs image sensing continuously while moving the lens. Since a problem with the former method is that it requires time since image sensing is performed after waiting for the lens to stop, the latter method is advantageous for speeding up the autofocus.
However, the following problems arise in a case where the latter method is employed when driving an image sensing element whose charge accumulation timings differ for a plurality of AF (auto focusing) areas. More specifically, when acquiring AF evaluation values at a plurality of positions in an image, the position of the focusing lens corresponding to the AF evaluation value of the AF area from which the image signal is first read differs from the position of the focusing lens corresponding to the AF evaluation value of the AF area from which the image signal is last read. As a result, a deviation occurs with respect to the position of the focusing lens that is judged to be in-focus for each AF area.